Recent studies have shown that the earth's ozone layer has suffered severe depletion in recent years. Ozone is recognized as the stratospheric component shielding against the harmful forms of ultraviolet (UV) radiation.
In general terms, harmful UV rays, particularly those originating from sunlight, which penetrate the upper atmosphere and reach the earth's surface, can be classified into:
1. the energy-rich UV-B rays (280-320 nm wavelength) which posses an intense physiopathological activity on human skin; these are absorbed just above the dermis and they are responsible for erythema and skin pigmentation, and
2. UV-A rays (320-400 nm wavelength) which penetrate deeper into the skin. The acute and harmful action of the UV-A on the skin is small when compared with that of UV-B, however, the chronic action of UV-A is now considered to be closely related to suntan, aging of the skin and the development of pigmentary blemishes.
Certain organic sunscreens whose molecules absorb the harmful UV rays have been proposed for use in mitigating the deleterious effects of UV radiation. However, these UV ray absorbers involve various problems in safety and the lasting effects thereof. On the other hand, various studies have been carried out on the absorption and scattering of UV rays, and a certain kind of inorganic powder has been known to have a great effect particularly for interrupting UV rays.
In spite of this and other prior proposals, there still exists a need for a highly efficient and thoroughly safe sun protection composition which has a wide spectrum of protection in the harmful UV region.
It is the principal object of the present invention to provide an inorganic oxide suitable for use in sunscreen compositions having improved UV ray-blocking properties. Various inorganic oxides, identified as Bolite compounds, are related art as described in the patent literature and in the published journals. Exemplary of these materials are Bolite-1, -2, -3 and -4 in U.S. Pat. No. 5,064,629, Bolite-7 in J. Mol. Cat. 68, (1991) 301-311 and Bolite-A, -B, -C, -D and -E in Materials Letters, 19 (1994) 213-216 by H. Asaoka. Some of these materials containing titanium are photocatalysts for cleavage of water in their metal-loaded form, but they cannot decompose water under illumination in their metal-unloaded form.
Generally, a superior UV preventing effect is observed in titanium dioxide which is the most potent photocatalyst. The active sunscreening agents must be chemically stable and in particular must be resistant to chemical and photodegradation when on the skin as well as resistant to absorption through the skin. Common titanium dioxide possesses catalytic activity, so that other cosmetic ingredients are liable to degrade.
A cosmetically acceptable sunscreen compound is accompanied by a second necessary component of the compositions such as zinc oxide, tin oxide, iron oxide, silica, mica, octyl methoxycinnamate and green tea as referred to in U.S. Pat. Nos. 4,820,508, 5,032,390, 5,215,749, 5,215,580, 5,234,682 and 5,306,486.
The present invention is concerned with a novel class of inorganic sunscreen materials, hereinafter designated as Bolite-S, having a composition expressed in terms of moles of oxides which may be written as: EQU mRO:vB.sub.2 O.sub.3 :wFe.sub.2 O.sub.3 :xSiO.sub.2 :yTiO.sub.2 :zH.sub.2 O
wherein "R" is selected from the group consisting of hydrogen, the alkyl groups containing 1-4 carbon atoms, and mixtures thereof, "m" is a value between 0 and about 1200, "v" is a value between 0 and about 500, "w" is a value between 0 and about 100, "x" is a value between 0 and about 200, "y" is a value between 1 and about 300 and "z" is a value of from 0 to about 300.
In accordance with the present invention, the Bolite-S compounds are prepared from the reaction mixtures containing five principal reactants; namely, titanium(IV) alkoxide, iron(III) alkoxide, silicon(IV) alkoxide, orthoboric acid (H.sub.3 BO.sub.3) and pyridine.
The reaction mechanism for the formation of the Bolite materials from titanium(IV) alkoxide and H.sub.3 BO.sub.3 in pyridine is described by H. Asaoka in Materials Letters, 19 (1994) 207-212 and 213-216.
The reaction is described as follows: EQU (OR).sub.3 Ti--OR+HO--B(OH).sub.2 .fwdarw.(OR).sub.3 Ti--OH+RO--B(OH).sub.2 ( 1) EQU (OR).sub.3 Ti--OH+RO--Ti(OR).sub.3 .fwdarw.(OR).sub.3 +TiO-Ti(OR).sub.3 ROH (2) EQU ROH+RO--B(OH).sub.2 +pyridine.fwdarw.HPy.sup.+ [(RO).sub.2 B(OH).sub.2 ](3)
wherein R is an alkyl group and HPy.sup.+ is protonated pyridine.
The reaction (1) in this system involves a transfer of an alkyl group from the alkoxide to orthoboric acid. The reaction (2) can be proceeded by the reactive precursors resulting from the first step of the reaction (1). The reaction (3) take place rapidly so that it is hard to detect alcohol in the reaction medium. The subsequent condensation reactions are possible to occur between the dimer resulting from the reaction (2) and a reactive species resulting from the reaction according to the equation (1).
The reaction can be written as follows: EQU (OR).sub.3 Ti--O--Ti(OR).sub.3 +HO--Ti(OR).sub.3 .fwdarw.(OR).sub.3 Ti--O--Ti(OR).sub.2 --O--Ti(OR).sub.3 +ROH
The successive reactions relating to the above reaction can be applied to all linear polymers.
In reality, the polymerization occurs simultaneously in two and three dimensions. In these cases, the branched unit will be introduced where the molar ratio of [H.sub.3 BO.sub.3 ]/[alkoxide] is greater than 1.
The prerequisite with respect to the solvent is a sufficient solubility of the alkoxide monomers and the reaction intermediates. Pyridine is a good solvent for metal alkoxides, oligomeric metal oxides and the pyridinium dialkoxydihydroxyborate (1-) complex (HPy.sup.+ [(OR).sub.2 B(OH).sub.2 ].sup.-).
The method just described for preparing the monolithic gels can be adapted to the polymerization reaction of plural metal alkoxides in the presence of orthoboric acid in pyridine to produce the multicomponent inorganic compounds.